The pure in heart are those people who not only do not sin openly, but who do not conceal unclean thoughts, desires and feelings in their hearts. The hearts of such people are free from attachment and infatuation with physical, earthly things. In general they are free from sinful passions caused by self-centeredness, egotism and pride. People with pure hearts unceasingly think about God.
In order to acquire a pure heart, it is necessary to observe the fasts proclaimed by the Church, and to guard oneself against gluttony, drunkenness, depraved spectacles and amusements, improper teachings and indecent books.
Purity of heart is far superior to simple sincerity. Sincerity requires only that a person be candid and single hearted in relation to his neighbor. But purity of heart requires complete suppression of depraved thoughts and constant remembrance of God and His holy commandments.
To the pure in heart God promises that they will see God. Here on earth they will see Him through Grace, mysteriously, with the spiritual eyes of their hearts. They can see God in His revelations, images and likenesses. In the future, eternal life, they will see God as He is (I John 3:2). Furthermore, since contemplation of God is a source of the highest blessing, the promise to see God is a promise of the highest degree of blessedness.